Many granular or fluent particulate materials are conveniently transported by truck, although they may also be transported by railcar, barge or by other means. Trucks that are used to transport such materials may include a tractor and an attached trailer having a tank or other container mounted thereon. Frequently, these trailers are referred to as pneumatic tank trailers because of the pneumatic method, involving gas- or air-entrainment, by which they may be loaded and unloaded. Materials that are generally transported in pneumatic tank trailers include agricultural products such as grain, corn kernels, beans, flour, sugar, peanuts and the like, and intermediate products for various industrial uses such as plastic pellets, coke, lime, silica gel, powdered acid resins, rare earth powders and powdered alumina, and many others.
Pneumatic tank trailers may have one or more product tanks or compartments, each of which is generally provided with an opening or access port. These access ports are provided for loading of the compartment with product and are usually located at the top of the compartment. Access ports are also usually large enough to allow a person to enter the compartment for inspection or cleaning. Access ports are usually circular and may have a diameter of about 20 inches. In addition, conventional access ports are frequently provided with a raised lip or collar around the periphery of the port. Each access port is also provided with a hatch cover assembly that permits the port to be closed or sealed. Because most of these pneumatic trailers are unloaded by a method which involves pressurizing the tank or product compartment, the hatch cover assembly is usually capable of maintaining a sufficient seal of the access port to maintain the pressure within the tank. Therefore, they are usually of relatively massive and heavy construction. Conventional hatch cover assemblies include a hatch cover that is hinged at one side so as to pivot about a horizontal axis, so that when the hatch cover is open, it extends vertically upwardly from the surface of the compartment. One such hatch cover assembly is described in U.S. Pat. No. 3,860,142 of Jurges.
Because of the method by which they operate, conventional hatch cover assemblies may not permit loading of the product into the trailer in a garage or other structure, because there may be insufficient clearance to permit the cover to be opened. In addition, conventional hatch covers may be difficult for some persons to operate, because most of the weight of the cover must be manually lifted upwardly in order to open it. It would be desirable, therefore, if a hatch cover assembly could be provided that could be opened in a garage or other structure having low clearance. It would also be desirable if such an assembly could be provided that would be easier than conventional assemblies for a person to operate.